Method of preparing glass fiber filter

A method for preparing a plasma or serum sample from a whole blood sample, by contacting a glass fiber filter which contains calcium, sodium or potassium components with an aqueous solution of an organic acid or a mineral acid to elute the calcium, sodium and potassium components in the glass fiber filter into the aqueous solution, separating the aqueous solution from the filter and washing the glass fiber filter with water to remove any residual aqueous solution adhered thereto and then filtering a whole blood sample through the thus treated fiber filter.